The present invention relates, in general, to solid state sensors, and more particularly, to solid state sensors having a micromachined capacitor structure.
Many micromachined devices are now well known, including force, acceleration, and pressure sensors. The term "micromachined" is used because the devices employ mechanical structures and geometries which are as small as a few tenths of a micrometer. The small dimensions are achieved by photolithographic and etching techniques similar to that used in integrated circuit manufacturing. Usually, many devices are manufactured on a single substrate. Often, a silicon substrate is used.
Most often, micromachined sensors use piezoresistive properties of silicon to generate a signal. Alternatively, capacitor plates can be formed on the substrate so that at least one capacitor plate can move with respect no another capacitor plate. The relative movement in response to pressure or acceleration changes the capacitance of the structure. This change in capacitance is detected as an output signal.
Unfortunately, prior micromachined capacitor structures suffer from a number of limitations which raise the cost of manufacture, limit accuracy, and preclude their use in many applications. For example, most capacitor structures currently available are only sensitive to acceleration that occurs vertically with respect to the surface of the device. This creates difficulty in packaging and mounting the device in many applications.
Prior capacitor structures were usually designed as cantilevers with one end anchored to a substrate and another end free to swing vertically in relation to a fixed capacitor plate. Capacitance of the device varied as the spacing between the cantilevers and the substrate became smaller. Unfortunately, capacitance changes non-linearly with spacing between capacitor plates, resulting in a non-linear response characteristic for the accelerometer.
What is needed is a micromachined capacitor structure and method for making it that is sensitive to lateral acceleration and provides a linear capacitance change with acceleration.